A reduction-type starter is disclosed in U.S. Pat. No. 5,557,976 (JP-A-7-293404). This starter has a planetary gear reduction device, which transmits rotation of a motor to an output shaft while reducing the rotation speed. A motor rotation shaft and the output shaft are arranged coaxially to each other through the reduction device.
The output shaft is formed with an accommodation recess around its radial center on the rear end facing the motor shaft. The top end of the motor shaft is inserted into the accommodation recess through a bearing. A ball is held between the top end surface of the motor shaft and the axial end surface (bottom surface) of the accommodating recess, thereby to receive thrust load.
In this starter, the output shaft has a flange with an increased diameter at its rear end. The flange is formed with pin holes into which pins are press-fit to fix planetary gears of the reduction device. The output shaft cannot be heat-treated, because press-fitting the pins into the pin holes are likely to cause cracks around the pin holes if the flange has been heat-treated. Therefore, the output shaft is normally not subjected to heat treatment but instead subjected to induction hardening only with respect to parts, which need hardening.
However, the output shaft is not so hardened at the inside of the accommodation recess. As a result, the wall of the accommodation recess contacting the ball gradually wears as the number of operations of the starter increases. The increase in the wear tends to move the output shaft to the motor side. As such the pins supporting the planetary gears interfere with a partition wall disposed between the motor and the reduction device, thus influencing the motor rotation. In addition, the increase in the wear instabilize rolling or movement of the ball. As such it becomes difficult to align the radial centers of the motor shaft and the output shaft which are coupled through the ball. Misalignment of the radial centers of the motor shaft and the output shaft produces unbalanced or uneven load on the planetary gear reduction device, thus causing loss in rotation and lowering of starter performance.